1. Field of the Invention
The present invention relates to a construction of a mold for use in an injection-molding optical data recording disk substrate or optical part such as a lens and more particularly to an injection-compression mold for implementing injection-compression molding by changing a clamp force in conformity with the condition in which a cavity is filled with a resin.
2. Description of the Prior Art
Optical data recording disk substrates as recording media, including optical audio disks, video disks, magneto-optic disks and the like, are high-quality, high precision molded products which require not only excellent optical properties but also dimensional precision. In other words, any of these molded products has to be free from sinkmarks, made to inherit the same configuration as that of a mold, immune from residual deformation and stress, provided with a uniform refractive index, particularly a smaller birefringence index as the birefringence index results from incident light and reflecting light which are out of phase.
In order to satisfy the aforementioned requirements, a number of proposals have heretofore been made. More specifically, Japanese Patent Laid-Open No. 73058/1976 discloses a means for improving a molding machine by conducting clamping on a multistage basis for compression; Japanese Patent Laid-Open No. 137625/1985 discloses a method of compressing a mold by providing a hydraulic cylinder between a stationary or movable plate and a mold part; Japanese Patent Laid-Open No. 137626/1985 discloses the provision of such a hydraulic cylinder between one side of the stationary plate or toggle support and the end of a tie bar, that side being fitted with no mold part; and Japanese Patent Laid-Open No. 171118/1985 discloses a method of optionally adjusting clamp force during injection molding by forming an extra-small gap on the outer periphery of a mold cavity.
With respect to a means for improving a mold, Japanese Patent Laid-Open No. 241115/1986 discloses a method of controlling a compression molding margin by providing a pressure-receiving plate for supporting a movable part with a hydraulic unit to push the movable part and inserting a space in between the tip of guide rod and pressure-receiving plate; Japanese Patent Laid-Open No. 109618/1987 proposes a method of continuously changing clamp pressure within a predetermined period of time; and Japanese Patent Laid-Open No. 137747/1987 discloses a method of improving heat treatment.
Of the aforementioned proposals, Japanese Patents Laid-Open Nos. 73058/1976, 137625/1985, 137626/1985 and 241115/1986 are intended to control the compressive clamp force in the case of an injection-compression molding machine using a toggle type clamp mechanism and have posed problems in that the molding machine and the mold tend to become complicated in construction and therefore costly.
On the other hand, the disclosures made by Japanese Patent Laid-Open Nos. 171118/1985 and 109618/1987 relate to an injection-compression molding machine using a direct pressure type clamp mechanism. Although the mold requires no special construction in order to directly control the pressure of a clamp ram for generating clamp force, the problem is that its response to compression is low, which also makes costly the control system for controlling the compressive force. In addition, it is necessary to control the compressive force during the process of filling the cavity with a resin or cooling it. Such control can hardly be possible with high reproducibility and quick response and besides large cost has been incurred to realize the control. Moreover, a small gap is normally provided between the abutting faces of the outer peripheries of the mold parts (hereinafter called PL planes) because compressing strokes have to be given during the injection-compression molding. For this reason, the movable part and the stationary part are cantilevered with their respective platens and they tilt under their own weight when the clamp force is horizontally directed. As a result, the lower section of the molded piece tends to become thicker than that of the upper section. The disadvantage is that optical parts such as optical data recording disk substrates and lenses are unable to retain uniform precise thickness.